GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

HRV for wet room exhaust: Will I have to build a motorized damper box myself?

mangler66 | Posted in Mechanicals on

I currently own a 2006 Lifebreath HRV 155cfm unit in SouthWestern Ontario. It was ducted well enough, but it basically pulls from too many areas of the house (kitchen, laundry room, 3 bathrooms). It does a good job of whole house ventilation, but basically it feels underpowered for wet room exhaust (1850sqft 2 story, with unit in basement, so some runs are also quite long, all flex).

In order to “fix” this, I had to adjust the room diffusers to prioritize the ensuite bathroom. I basically shut down all the other diffusers except that one. Steam evacuation is now adequate, but still not stellar.

I am now planning a new house, and the unit I am looking at (Enerboss airhandler/HRV combo) now has a dedicated wet room exhaust port with additional fan, which should help a lot. I would like to take it a step further, and build a junction box going to all 3 bathrooms, with a motorized damper set so when the specific boost switch in a bathroom is triggered, it not only starts the boost fan, but shuts off access to the other 2 bathrooms to give me the full 160cfm to the bathroom being used. Did someone already figure this out (can I buy this pre-made) or will I have to build it myself?

Other details that I hope will help:

Metal ducting for the HRV, as much as feasible
Better positioning of the HRV exhaust in bathrooms (ie in showers vs in bathroom currently)
Walk in showers with a “sealed” design (current glass shower has walls that do not go to the ceiling, with a 1 foot opening at the top that leaks massive amounts of moisture in the bathroom during a shower.)

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Mai Tai,
    I don't recommend installing any motorized dampers in your exhaust ducts. I'm not aware of any HRV or ERV manufacturers that allow this approach. As you probably know, you have to install your ventilation equipment in compliance with the manufacturer's instructions if you expect any warranty coverage.

    Your problem, and a variety of solutions, are discussed in this article: Does a Home with an HRV Also Need Bath Fans?

    -- Martin Holladay

  2. charlie_sullivan | | #2

    If you are concerned about bathroom moisture to that extent, you might consider the effect of the shower design. Fully enclosing the shower will drastically reduce the moisture put into the air. The humidity in the stall reaches 100%, and then no more moisture goes into the air. That small volume of 100% humidity air still gets released when you open the door, but when it mixes with the larger volume of air in the room, the quantity of moisture released is much smaller than it would be with an open-top stall. You then need to leave the shower door open for it to dry, but it drys fast with the overall room humidity low.

    One commercial product:

  3. WEG | | #3

    Answer #2 is correct. I use HRV only for bath exhaust and it was inadequate until we installed a shower door. I have four HRV returns, 3 bathrooms and 1 near kitchen and supply air dumps into duct system return. If ever do this again (which is only possible with another partner and I don't plan on that) I would pull from each bedroom closet and put small bath fans in bathrooms.

  4. WEG | | #4

    I have several builders that vent the bathoom exhaust into the HRV return. There is no control communication between the two units. So when the bath fan turns on you get 100 cfm pushed through the HRV core without knowing if the HRV is running. It is a big house so it may be continuously running. Builder and his HVAC sub really like this method. If anyone can shed light on whether this is a good idea or not let me know,

  5. STEPHEN SHEEHY | | #5

    Our master bath shower has no partitions at all. It's completely open to the rest of the bathroom. Room is about 6' x 14.' We run the hrv boost when showering and for at most ten minutes after. By then, the moisture has dissipated. We do leave the bathroom door open a bit. If bathroom door is closed, we need an extra ten minutes to get rid of the moisture.
    I'd think a fully enclosed shower would work as suggested, but skipping the enclosure is cheaper to build and avoids the glass cleaning chore and the messy joint at floor and ceiling.

  6. GBA Editor
    Martin Holladay | | #6

    It's not clear whether or not you have read the article I linked to: Does a Home with an HRV Also Need Bath Fans?

    Your description of a ventilation technique is also a little unclear. You wrote that some builders "vent the bathoom exhaust into the HRV return. There is no control communication between the two units. So when the bath fan turns on you get 100 cfm pushed through the HRV core without knowing if the HRV is running."

    If you mean that each bathroom has a dedicated bathroom exhaust fan, and that the exhaust ducts from these exhaust fans are ducted to an HRV, then I would say (a) that is not a good idea, and (b) that approach is not allowed by the installation instructions of any HRV manufacturer that I know of, and (c) you should read the article I linked to.

    -- Martin Holladay

  7. charlie_sullivan | | #7

    Walter, if you use a bath fan to push air into the HRV return when the HRV is off, I would expect a large fraction of that air to make it's way back into the house by traveling backwards through the other exhaust ducts. The humidity and smells you are hoping to remove could be dumped into the kitchen and other bathrooms.

    Doing that when the HRV is running could possibly be balanced just right so that the flow does not reverse in any of the exhaust vents when a single fan is running, but it would be hard to make that work with multiple exhaust fans, without a control system that actively modulated the HRV fans as well as the exhaust fans. That could be an interesting future product idea, but I don't think it's a feasible approach with the equipment on the market now.

  8. mangler66 | | #8


    I have read the article you referenced. The consensus is that HRVs can be used as standalone bath fan replacements. Which I agree with fully. My goal now is to make them comparable to a dedicated bath fan with a 6 foot or less straight duct to the outside.

    While I understand why you would recommend against booster fans or dampers (not approved by manufacturers), I am a big fan of the why, as in why not. An HRV really is an aluminum box with a fan and an aluminum core, so besides the potential maintenance issues of the damper approach (mitigated if you keep the damper box in the utility room with easy access), I don't see any potential issues. All a booster fan at the exhaust site will do is help you overcome the restriction of a long duct line, and maybe cost you a few points in efficiency. Of course limiting the cfm of the booster to half or 2/3rds of the max HRV cfm will ensure you don't get any blowback through other open ports. Of course the switch has to turn on the booster fan AND the HRV in boost mode, simultaneously. This would address Walter's issue also. The system he described (no link between fan and HRV) sounds more like a humidity distributor to me...

    CHRSULL, thanks for the suggestion on the shower dome. I am off to find one (hopefully closer than New Zealand) for my current shower.

  9. mangler66 | | #9

    Looks like I won't have to build one myself. There is at least one company who gets it : motorized dampers to extract stale air/moisture from specific zones, on demand. I suspect there will be a lot more coming:

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |